Track error count in issues.go alongside the existing issues slice.
Add hasErrors() helper. In main.go, call osExit(1) after writeRunLog()
if any [ERROR] entries were recorded, so CI steps correctly fail when
errors occur (e.g. GitHub API 403 rate-limit hits in the macOS job).
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>